The present invention relates to a method of bleaching cellulose pulp with ozone, and specifically to a method of bleaching pulp with ozone in a foam-like mixture.
Pulp for the paper and pulp industry must often be bleached in order to produce an end product of suitable high-quality. The most commonly used bleaching agents today are chlorine and oxygen. There is a tendency to avoid the use of chlorine or at least limit its use to a minimum because of damage caused thereby to the environment. Oxygen is a suitable bleaching agent but its reaction selectivity is not always adequate so that other additional chemicals must be used. For these reasons, new bleaching agents have been sought. Ozone is one of them.
Ozone bleaching has been extensively studied in laboratory and pilot scale. Ozone has proved to be a satisfactory bleaching agent but also expensive and difficult to use as the consistency of the pulp to be bleached has to be very low or very high due to the high reactivity of the ozone. For example, at low consistencies, i.e. below 5%, ozone is dissolved in the water and thus satisfactory transfer of mass between the ozone and the fibers in the water is achieved as the ozone containing water can freely flow between the fibers. It has also been found out that ozone, being a gaseous substance, reacts well directly with a dry fiber surface which presupposes that the consistency is so high, in most cases over 30%, that there is practically no water on the surface of the fiber or between the fibers. In these circumstances the ozone containing gas can freely flow between the fibers.
On the other hand, to assure proper pumpability of the suspension, a certain amount of free water in the suspension must be present. For environmental and other reasons, it is desirable to keep this amount of water as small as possible. Accordingly, these factors define the consistency range which is optimal for both the apparatus and the environment and which lies between about 5 and about 25%. However, in this particular consistency range ozone cannot contact the fibers in a satisfactory way as there is relatively little liquid in the suspension which is bound in the spaces between the fibers and does not move freely in the suspension, and as ozone, being a gaseous substance, cannot, due to the state of the suspension, move freely within the fiber suspension.
The aforesaid problems have been solved by providing a method for bleaching pulp with ozone at a consistency range of about 5 to about 25%. According to the invention, conditions for good mass transfer are created even under conditions in which gas or water cannot move freely throughout the fiber suspension. Hence, the present invention provides bleaching the fiber suspension in the form of a foam-like mixture. Preferably, the fibers are pumped with a medium consistency pump (MC pump) to a fluidizing mixer, in the mixer oxygen and ozone containing gas is mixed into the pulp, contacting the ozone gas which serves as the bleaching agent with the fibers of the pulp by mixing the gas into the pulp, and discharging the fiber suspension from the mixer, preferably, into a reaction vessel.
In a preferred embodiment, the bleaching process is performed in at least two steps and residual gas is removed after the first step or stage preferably in a reaction vessel. Prior to the second stage, additional oxygen and ozone containing gas is added to a second mixer provided downstream of the first mixer and reaction vessel.